The present invention relates to catalysts suitable for the polymerisation of olefins and in particular to supported metallocene catalysts providing advantages for operation in gas phase processes.
In recent years there have been many advances in the production of polyolefin homopolymers and copolymers due to the introduction of metallocene catalysts. Metallocene catalysts offer the advantage of generally a higher activity than traditional Ziegler catalysts and are usually described as catalysts which are single site in nature. There have been developed several different families of metallocene complexes. In earlier years catalysts based on bis (cyclopentadienyl) metal complexes were developed, examples of which may be found in EP 129368 or EP 206794. More recently complexes having a single or mono cyclopentadienyl ring have been developed. Such complexes have been referred to as xe2x80x98constrained geometryxe2x80x99 complexes and examples of these complexes may be found in EP 416815 or EP 420436. In both of these complexes the metal atom eg. zirconium is in the highest oxidation state.
Other complexes however have been developed in which the metal atom may be in a reduced oxidation state. Examples of both the bis (cyclopentadienyl) and mono (cyclopentadicnyl) complexes in which the metal has the oxidation state of +2 have been described in WO 96/04290 and WO 95/00526 respectively.
The above metallocene complexes are utilised for polymerisation in the presence of a cocatalyst or activator. Typically activators are aluminoxanes, in particular methyl aluminoxane or compounds based on boron compounds. Examples of the latter are borates such as trialkyl-substituted ammonium tetraphenyl borates. Catalyst systems incorporating such borate activators are described in EP-561479, EP 418044 and EP 551277.
The above metallocene complexes may be used for the polymerisation of olefins in solution, slurry or gas phase. When used-in the gas phase the metallocene complex and/or the activator are suitably supported. Typical supports include inorganic oxides eg. silica or polymeric supports may alternatively be used.
A preferred gas phase process for polymerising olefins in the presence of a metallocene catalyst is one operating in a fluidised bed. In such processes the molecular weight of the polyolefin produced by the metallocene complex is determined by the competing rates of chain propagation, chain termination and chain transfer. These rates are in turn determined by the intrinsic kinetics of the catalyst and the reaction environment. In order to produce commercially interesting polyolefins the catalysts must make a high molecular weight polymer. Furthermore at commercial reactor conditions the catalyst must make a molecular weight that exceeds that of commercially interesting polymers such that the molecular weight can be controlled at the desired value with a chain transfer agent such as hydrogen.
WO 98/27119 describes supported catalyst components comprising ionic compounds comprising a cation and an anion in which the anion contains at least one substituent comprising a moiety having an active hydrogen. In this disclosure supported metallocene catalysts are exemplified in which the catalyst is prepared by treating the aforementioned ionic compound with a trialkylaluminium compound followed by subsequent treatment with the support and the metallocene. When used in the gas phase such supported catalysts are extremely active but the molecular weight of the produced polymer is lower than that required for commercial operation.
WO 98/27119 also describes a method for activating a substantially inactive catalyst precursor comprising (a) an ionic compound comprising a cation and an anion containing at least one substituent comprising a moiety having an active hydrogen, (b) a transition metal compound and optionally, (c) a support by treatment with an organometallic compound thereby forming an active catalyst.
We have now surprisingly found that by careful choice of the preparative route to the supported catalyst and the nature of the metallocene complex itself, catalysts based on certain bridged bis (cyclopentadienyl) zirconium complexes and activators based on boron compounds having at least one substituent comprising a moiety having an active hydrogen may be used in gas phase processes to control the chain transfer rate in order to produce polymers in commercial gas phase reactors with the desired molecular weight distribution. The advantage of the invention is such that a wide range of molecular weights and molecular weight distributions may be accessible in the same gas phase process by careful choice of the supported catalyst and it""s preparation. Melt index control and improved opticals may also be easier with the supported catalysts of the present invention.
Thus according to the present invention there is provided a process for the preparation of a supported metallocene catalyst comprising the following steps:
(i) mixing together in a suitable solvent
(a) a metallocene of formula: 
wherein Cp is a cyclopentadienyl ligand,
each R group represents an alkyl or an aryl substituent or two R groups may be joined together to form a ring,
Z is a bridging group comprising an alkylidene group having 1-20 carbon
atoms or a dialkyl silyl or germanyl group,
M is zirconium in the +2 formal oxidation state
D is a neutral xcex74 bonded diene group having up to 30 non-hydrogen atoms which forms a xcfx80-complex with M,
and n and m are the same or different and equal 0-4, and
(b) an ionic compound comprising a cation and an anion having up to 100 non-hydrogen atoms and containing at least one substituent comprising a moiety having an active hydrogen, and
(c) optionally a support material,
(ii) treating the mixture obtained in (i) with an organometallic compound, and
(iii) addition of a support material if not added in step (i), and
(iv) removing the solvent.
A suitable solvent for the supported catalyst preparation is toluene.